Tuesday, March 30, 2010
Eat Fat for Weight Loss
As most low-carbers already know, eating fat produces satiety. But in some cases, eating fat also helps a dieter lose weight.
It turns out that the chain length of the fatty acids in the triglyceride is an important factor in choosing a fat that promotes weight loss. Most of the fats found in a normal diet will contain long-chain fatty acids. That is, most of the triglycerides we eat will have fatty acids that contain between 13 and 22 carbons. These long-chain fatty acids are digested in the gut, where they are packaged into chylomicrons. The chylomicrons are moved into the lymphatic system and eventually enter the blood at the left subclavian vein in the upper chest. (A review of the process can be found here.)
When the chylomicrons reach the blood, the long-chain fatty acids in them can be absorbed by any cell, including fat cells, that contain lipoprotein lipase. Once these fatty acids are absorbed into a fat cell, they are still available for later mobilization into the blood via hormone-sensitive lipase. But in insulin-resistant individuals, the activity of hormone-sensitive lipase is down-regulated by high insulin levels. In those people, stored fat tends to remain in storage.
Medium-chain fatty acids contain from 6 to 12 carbons. (In a normal diet, the most common source is probably butter, which contains about 10% medium-chain fatty acids. For those who shop the health food aisles, another source is coconut oil, containing about 66% medium chain fatty acids.) Medium-chain fatty acids are processed differently in the gut. Because they are more water-soluble, they tend not to be packaged into chylomicrons. Instead, they are absorbed from the gut directly into the blood as free fatty acids. Medium-chain fatty acids are bound to serum albumin in the blood, and in that form they travel to the liver where they are used primarily for energy production. Some are converted to ketones that are in turn used for energy by many of the cells of the body.
A 1996 review article by Bach et al. discussed the fact that, compared with long-chain triglycerides, medium-chain triglycerides have more rapid delivery to the liver, higher oxidation rates, poorer rates of incorporation into fat cells, and greater control of satiety. However, there were some counteracting factors that suggested that eating medium-chain fatty acids might not produce the expected reduction in body weight.
After that review article was published, Marie-Pierre St-Onge and her colleagues began studying the effect of human diets that were either rich in medium-chain triglycerides or rich in long-chain triglycerides. The medium-chain triglyceride oil contained primarily caprylic (8 carbons, saturated) and capric (10 carbons, saturated) fatty acids. The long-chain triglyceride oil was olive oil, which contains primarily oleic acid (18 carbons, monounsaturated).
In a randomized crossover controlled feeding trial published in 2003, energy expenditure was measured before and up to 5.5 hours after eating a breakfast meal. Although both groups saw increases in fat oxidation and energy expenditure following the meal, the medium-chain triglyceride group saw larger increases at some though not all of the timepoints after the breakfast meal. The medium-chain triglyceride group also saw a trend toward lower energy intake at the subsequent lunch meal. Not surprisingly, over the four-week duration of the study, the medium-chain triglyceride group saw a significant loss of total adipose tissue of about 1.8 pounds. The reduction in adipose of the olive oil group did not reach significance.
In 2008 Dr. St-Onge and colleagues performed a 16-week double-blind non-crossover weight loss study in overweight men and women. Once again, the groups were divided according to diets containing either medium-chain triglycerides or olive oil. Women consumed 1500 calories per day and men consumed 1800 calories per day, with about 12% of these calories as the prescribed study oil. At the end of the study, those who consumed medium-chain triglyceride oil had lost about 3.7 more pounds of body weight than those in the olive oil group. The loss of total fat mass was also about 3.2 pounds greater in the medium-chain triglyceride group compared with the olive oil group.
These findings are consistent with those of other investigators, both for animal models of obesity and for humans. In 2007 a group in China performed a pilot study to see if other health parameters are affected with the ingestion of medium-chain triglycerides. For ninety days, forty moderately overweight type 2 diabetic patients were given either 18 grams per day of medium-chain triglycerides or 18 grams per day of corn oil. The medium-chain triglyceride group showed a reduction in body weight, a reduction in waist circumference, a decline in serum cholesterol, an increase in serum C-peptide and a reduction of insulin resistance.
The studies discussed in this blogpost are not definitive, and much more research will be necessary to see if medium-chain triglycerides are an effective tool for reducing obesity. Nevertheless, it is encouraging to see that, at least in an experimental setting, these fats are able to decrease fat mass in both overweight men and women over time.